Rotating hole boring tools, generally known as boring bars, are used to accurately size a bore that has already been drilled or cast in place in an object such as an engine block. A cutting point or insert is fixed to a solid body cutter body which is releasably gripped in a holder that is in turn powered by a machine spindle. The cutter body can thus be removed from the holder and spindle for service or replacement. The cutting point may have a single, fixed radius relative to the axis of rotation, in which case it can bore only a single size hole. With wear or slippage, the cutting point cuts an inaccurate bore, generally undersized, but possibly oversized. Therefore, boring bars in which the cutting point can be adjusted radially in or out on the bar, and thus change the effective cutting radius relative to the spindle axis, are commercially available. Generally, these are used not so much to cut a range of hole sizes as to provide adjustment, with use and wear, but to keep the nominal hole size accurate.
A common adjustment scheme provides a dial mechanism located between the cutting point and bar that shifts the point in or out continuously. Some kind of threaded clamp has to be released and then retightened to make the adjustment. An example may be seen in U.S. Pat. No. 4,396,319 to Miles. The advantage of a continuously adjustable boring bar is also its inherent drawback, in effect. The clamp that allows the adjustment to be can also slip in use, and the cutting point is not as rigidly or stiffly held as it is a non-adjustable tool. Tool chatter and hold size inaccuracy can result. Furthermore, the very act of adjusting a cutting point by releasing a clamp means, moving it, and reclamping it is fraught with potential inaccuracy. The operator has to somehow hold the point in the proper position during reclamping, and it can shift.
A different adjustment scheme leaves the cutting point solidly and immovably secured to its boring bar, as in a standard boring bar, but retains the boring bar within a holder sleeve that is eccentric or offset relative to the main axis of spindle-holder rotation. The bar can be released, turned continuously to any angular position relative to the holder and, and reclamped into its new position. Because of the axis eccentricity, turning the bar within the sleeve changes the cutting point's effective cutting radius relative to the main axis of spindle rotation, just as directly moving the cutting point on the bar would do. However, since there is more potential surface area of physical contact between the boring bar and its holder than there is between the small cutting point itself and the boring bar, such a scheme is potentially more solid.
An example of the eccentric bar type of adjustment may be seen in U.S. Pat. No. 4,057,027 to Adamson et al. It is still not as solid or accurate as a non-adjustable boring bar. In the embodiment disclosed, the boring bar and holder have abutted circular flanges, one of which has dial typed gradations engraved on it. A solid, removable key fitted between the abutted flanges keeps the boring bar solid and rigid to the holder as an initial or rough cut is made. For later adjustment to make a finer cut, the solid key is removed, and two other clamps are released in order to allow the bar to be turned. One clamp is an exterior leg type clamp hooked over the boring bar flange, which does not fit down into any indentations, and so cannot be relied on to provide any real solidity. What the exterior clamp does is hold the flanges together temporarily while a secondary, internal cam clamp is tightened. The secondary clamp consists of a threaded bolt and a knurled wedge that pulls radially inwardly against splines on the boring bar shank to clamp it in place. While more solid than a movable point type of adjuster, it still suffers from the same inherent problems. That is, the flange has to be held temporarily in its adjusted angular position before the cam clamp is tightened down, and is subject to slippage. Furthermore, the radially engageable teeth on the wedge do not provide a lot of engaged surface area between it and the bar shank to prevent chattering during operation. Threaded bolts are also inherently subject to backing out. And, of course, the adjustment process is complicated in that it requires the loosening and retightening of two bolts, not counting the removal of the slotted key initially. These operations are all extra operations, above and beyond the basic operations of releasing, turning and regripping the boring bar in the holder.